Fig. 3: Changes in remaining carbon budgets due to socioeconomic uncertainty affecting future non-CO₂ forcing.

a Remaining carbon budgets as a function of future (\(f_{nc}^ \ast\)) and current (\(f_{nc}\)) non-CO₂ forcing fractions. Isolines (in GtCO₂) indicate the remaining budget associated with each combination of \(f_{nc}\) and \(f_{nc}^ \ast\), and the yellow star marks the median budget for the “main case” shown in Fig. 2. Circle markers indicate the pairs of \(f_{nc}^ \ast\) and \(f_{nc}\) values from each net-zero CO₂ scenario in the SR1.5 scenario database, with their respective histograms shown along the x and y-axis. In both cases, we used a 30-year average of forcing fractions, covering the 30 years prior to 2019 (for \(f_{nc}\)) and prior to the year of net-zero CO₂ emissions (for \(f_{nc}^ \ast\)). The black line shows the linear fit (ordinary least squares) to all the circles, with a ±0.05 offset in the intercept (dashed lines). The resulting remaining budgets for each offset are shown in b, representing the effect of scenario uncertainty on the range of \(f_{nc}^ \ast\) values. b Changes in the median, likely and 5–95% remaining budgets as a result of an increase or decrease in the difference between the current and future non-CO₂ forcing fraction (coloured bars) compared to the assessed scenario uncertainty effect in SR1.5 (dark grey bars).